1. Field of the Invention
The present invention relates to an apparatus for projecting a slide or video image onto a screen to display it and, more particularly, to an apparatus which can automatically and satisfactorily maintain a good focusing state of an image on a screen, and is started when the apparatus is set.
2. Related Background Art
Conventionally, projection type display apparatuses such as a slide projector and the like are known. In recent years, some projection type display apparatuses form an original image to be projected using, e.g., a liquid crystal light valve.
In these projection type display apparatuses, in order to release a user from a cumbersome focusing operation, a system in which a focus detector is arranged outside the projection type display apparatus to automatically perform a focusing operation is also known.
However, this system suffers from the following problem. That is, a focus detection projection light source must be separately arranged. In addition, since the system is often used in a state wherein the screen is arranged not to be perpendicular to the optical axis of a projection lens, when a focus detection position on the screen is separated away from the optical axis position, focus detection cannot be normally performed.
The system has a structure that can prevent a projection lens from being adversely influenced by, e.g., heat from a light source. However, when the system is used for a long period of time, the refractive index of the projection lens changes due to heat, and the focal point position may be moved. The system cannot properly cope with such a problem.
A conventional projection apparatus such as a slide projector, an overhead projector, or the like generally employs a manual focusing apparatus. In this focusing apparatus, an arbitrary photographing magnification is set, and thereafter, the focusing operation of a photographing lens is manually performed to attain an in-focus state. On the other hand, as an automatic focusing apparatus for a projection apparatus, Japanese Patent Application No. 47-19077 proposes a system shown in FIG. 48 as an auto-focus system for a slide projector.
In this slide projector, as shown in FIG. 48, a slide film 2 is arranged on the focal plane of a projection lens 1, and an image on the film 2 is projected onto a projection surface 3 arranged on the front surface of a photographing lens. In this arrangement state, in only the first projection operation, the projection lens 1 is moved in the optical axis direction to perform a focusing operation, so that an image is focused on the projection surface 3. A light source unit 4 and a light-receiving unit 5 are arranged on two sides of the film 2, so that a light flux emitted from the light source unit 4 is reflected by the surface of the film 2, and is incident on the light-receiving unit 5. The light-receiving unit 5 can detect the position of the incident light flux, and stores the position of the light flux reflected by the surface of the film 2 as an in-focus position of the film when the projection lens 1 is initially moved in the optical axis direction to perform the focusing operation. When another film is set, the position of a light flux reflected by the surface of this film is detected, and when the detected position is different from the stored film in-focus position, the film 2 is moved in the optical axis direction by a film position driving device 6 to adjust the detected position to the stored film in-focus position. With the above-mentioned operation, when films are changed, an in-focus state can be automatically maintained.
Japanese Laid-Open Patent Application No. 1-204010 proposes a focus system in which a light projection/receiving device having a special-purpose optical system for focus detection is arranged to measure a distance to a projection surface in addition to a focusing optical system for projecting an image reproduced on a liquid crystal surface onto the projection surface, and the focusing optical system for projecting an image reproduced on the liquid crystal surface onto the projection surface is adjusted based on the data to obtain an in-focus state.
A projector as a typical projection type display apparatus may have an arrangement shown in FIG. 50. In FIG. 50, the projector comprises a light source 11, an object 12 to be projected, a screen 19, a driving system 15 for driving a projection lens 16, and a motor 14 for driving the driving system 15. In order to perform focus adjustment, light projected by a light projection/receiving unit 18 is reflected by a half mirror 13, and is projected onto the screen 19 through the projection lens. Furthermore, the projected light is reflected by the screen 19, and is measured by the light projection/receiving unit 18 along the same optical path.
In order to stabilize a projected image, the focus adjustment is performed when a power switch is turned on or when an auto-focus switch is operated.
However, in the above-mentioned prior art, when the temperature of the projection lens changes during projection, and the focal point position is shifted, the auto-focus switch must be operated, resulting in cumbersome operations.
As one kind of optical equipment, a projection apparatus which incorporates an auto-focus device is known. As an invention associated with such a projection apparatus, the invention described in Japanese Laid-Open Patent Application No. 1-204010 is known. The invention of this application allows a simple operation of the projection apparatus with the auto-focus device. That is, an auto-focus operation is performed by detecting that the projection apparatus is set.
However, once the above-mentioned projection apparatus is set, if a person touches and moves the apparatus, the focusing operation must be manually performed. Therefore, the apparatus is not easy to operate or use, and automatic functions are not satisfactory.